What does "ywis" mean?

Ywis: Certainly; most likely; truly; probably. [Obs. or Archaic] "Ywis," quod he, "it is full dear, I say." Chaucer. She answered me, "I-wisse, all their sport in the park is but a shadow to that pleasure that I find in Plato." Ascham. A right good knight, and true of word ywis. Spenser. Note: The common form iwis was often written with the prefix apart from the rest of the word and capitalized, as, I wis, I wisse, etc. The prefix was mistaken for the pronoun, I and wis, wisse, for a form of the verb wit to know. See Wis, and cf. Wit, to know. Our ship, I wis, Shall be of another form than this. Longfellow.